In a wireless communication system, a base station may serve user equipment devices (UEs) such as cell phones, wirelessly-equipped computers, tracking devices, embedded wireless modules or the like (whether or not actually “user” operated) over an air interface. In practice, the air interface may define a continuum of transmission time intervals (TTIs) in which communications may pass on a downlink from the base station to the UEs and on an uplink from the UEs to the base station. In particular, in each TTI, the air interface may define a quantity of air interface resources, such as physical resource blocks (PRBs) or the like on which data may be modulated for transmission using an appropriate modulation and coding scheme.
When a UE is served by a base station in such an arrangement and the UE has data to communicate over the air to the base station, the UE may transmit to the base station a scheduling request asking the base station to assign air interface resources for uplink data transmission to the base station. Considering various factors, such as the extent of data that the UE has buffered for transmission, the UE's current air interface signal strength, the UE's available transmission power, and current load on the air interface, the base station may then allocate certain uplink air interface resources as a transport block for use by the UE to engage in uplink data transmission, providing a directive to the UE indicating the allocated resources. And the UE may then transmit data to base station on the allocated air interface resources.
In such a system, the base station and UE may also engage in a hybrid automatic repeat request (HARQ) process to manage re-transmission from the UE to the base station in the event of error. In a representative HARQ process, the UE may include with its transmission to the base station a cyclic redundancy check (CRC) that is computationally based on the transmitted data, and, upon receipt of the transmission, the base station may compute a CRC and determine whether it matches the CRC provided by the UE. If the CRC does not match (i.e., a CRC check fails) or if the base station otherwise determines that it has not completely received the data at issue (e.g., if the base station does not receive the scheduled transmission at all), the base station may then transmit to the UE on the air interface a HARQ negative acknowledgement (NACK), and the UE may then responsively re-transmit the data to the base station. In a representative system, the UE may engage in this re-transmission using the same air interfaces that the base station earlier allocated to the UE, but this time in a later TTI. On the other hand, if the CRC matches or the base station otherwise determines that it has completely received the data at issue, then the base station may transit to the UE a HARQ positive acknowledgement (ACK), which may let the UE know that the UE can proceed with a next data request/transmission rather than re-transmitting.
An example HARQ process may also involve the UE including with its transmission to the base station some forward error correction (FEC) bits that the base station can use as a basis to uncover the transmitted data in the event of an error. With such an arrangement, if the base station detects that the CRC does not match, the base station may try to apply the FEC bits to uncover the data (e.g., to get the CRC to match) and, absent success, may then send a HARQ NACK to the UE to trigger re-transmission. Further, the initial transmission and each re-transmission by the UE to the base station may include the same bits of data and perhaps different FEC bits if any, or may include different portions of the data bits and perhaps different FEC bits if any, but may still be considered re-transmission of the data. In practice, the base station may then work to complete its receipt of the UE's data transmission, perhaps by combining together the initial transmission with one or more re-transmissions (e.g., to piece together the intended data transmission and/or to apply various FEC bits to uncover the intended data transmission), or by ultimately receiving a successful transmission of the data and having it replace one or more previously received erroneous transmissions. Other arrangements are possible as well.
Through this process, once the base station completely receives the scheduled data transmission from the served UE, the base station may then forward the data via a backhaul link, such as a bearer tunnel established for the UE, to a switch or gateway, for transmission of the data turn onto a transport network such as the Internet.